Problem: Solve for $n$ : $4 = n + 13$
Explanation: Subtract $13$ from both sides: $ \begin{eqnarray} \\ \hphantom{4 {- 13}} &\hphantom{=}& \hphantom{n} \\ 4 &=& n + 13 \\ \\ {-13} && {-13} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 4 {- 13} &=& n \\ \end{eqnarray} $ Simplify. $ n = -9$